At present, under the circumstance of the coexisting 2G and 3G networks, the requirement for antennas which are compatible for 2G and 3G networks are continuously increasing. With the development of communication technology, higher performances of multiple band antennas are also desired.
Basing on the above development tendency, the design that two pairs of cross-polarized dipoles form in the shape of square or circle is commonly applied in the present market. U.S. Pat. No. 6,333,720B1 disclosed an antenna, of which the low band radiation element module included two pairs of cross-polarized dipoles arranged like a dipole square. High band radiation elements are embedded between low band radiation elements to achieve the performance of multiple band antennas.
In the design of U.S. Pat. No. 6,333,720B1, there are some defects in the low band radiation element and its multiple band antennas as following: (1) the linear dipoles have a big dimension of dipole square, which degrades the performance of high band radiation between low band radiation elements. In addition, the coupling between low band radiation elements degrades its electrical performance. (2) The structure of the balun is linear, which makes low band radiation element close to the high band, and the impedance and pattern of the high band radiation elements is effected by the low band radiation elements, which causes lower electrical performance and bad pattern.
Compared with U.S. Pat. No. 6,333,720B1, the design in Chinese Patent published No. CN201134512Y had some improvements. But it still had some defects as following: (1) since the high band radiation element is embodied in low band radiation element to achieve multi-band antenna, the high band radiation element is positioned near the low band balun, which degrades the VSWR (Voltage Standing Wave Ratio) and radiation performance of high band radiation element. (2) Although the design reduced the radiation dimension, all the dipoles at one end are bent downwards, which degrades the performance of high band radiation elements. (3) Different size of dipoles, specially the end thereof being enlarged to expand the operation band, also increases the difficulty of manufacturing and decreases the reliability of the radiation element.